Enhancing the softness of paper products such as tissue and towelling is desirable. Softness is the tactile sensation a user perceives as the user holds, rubs or crumples a particular paper product. This tactile sensation is provided by a combination of several physical properties including the bulk, stiffness and stretchability of the paper.
Creping, a process which is well known in the art, is a means of mechanically foreshortening a fibrous structure in the machine direction in order to enhance the softness, bulk and stretchability of the paper. Creping is generally accomplished with a flexible blade, known as a creping blade, which is placed against a drying surface such as a Yankee dryer. The fibrous structure adheres to the Yankee dryer as it contacts the dryer surface. The fibrous structure travels along the surface of the Yankee dryer until it is removed by the creping blade. The degree to which the fibrous structure adheres to the Yankee dryer prior to creping is a key factor in determining the degree of softness, bulk, and stretchability exhibited by the fibrous structure after creping.
Creping aids are generally applied to the surface of the Yankee dryer to further facilitate the adhesion/creping process. It is desirable to deposit the creping aid on the surface of the Yankee dryer. The level of adhesion of the fibrous structure to the Yankee surface is of importance as it relates to the control of the fibrous structure in its travel from the creping blade to the reel of the paper machine. Fibrous structures which are insufficiently adhered to the surface of the Yankee dryer are generally difficult to control and often result in quality problems at the reel such as wrinkling, foldovers and weaved edges. Poorly creped paper affects the reliability of the entire papermaking process and subsequent converting operation.
The level of adhesion of the fibrous structure to the Yankee surface is also of importance as it relates to the drying of the fibrous structure. Higher levels of adhesion permit better heat transfer. This enables the fibrous structure to dry faster thus allowing the operation to run at higher speeds.
It has been observed on paper machines utilizing through-air drying, that less creping aid tends to deposit on the dryer surface compared to paper machines utilizing conventional press sections. A through-air dried fibrous structure tends to be transferred to the Yankee dryer at a higher consistency than a fibrous structure which is conventionally wet pressed. Additionally, due to limited knuckle area, a fibrous structure which is through-air dried, will have a smaller area of contact with the nip of the Yankee dryer pressure roll than a fibrous structure which is conventionally wet pressed. Thus, through-air dried fibrous structures or other papermaking processes which yield structured paper tend to have poorer adhesion than fibrous structures which are conventionally wet pressed.
Prior to the development of creping aids, adhesion of the fibrous structure to the dryer surface was accomplished through the presence of naturally occurring hemicellulose present in the paper fiber. Hemicellulose deposits were observed forming on the surface of the dryer as a result of the evaporation of water from the fibrous structure. The hemicellulose deposits were found to contain small fiber fragments picked out of the fibrous structure. It was noted that these deposits resulted in the formation of a heavy film on the surface of the Yankee dryer and subsequent poor crepe quality. For those paper machines utilizing through-air drying, it was further observed that the formation of hemicellulose deposits on the surface of the Yankee dryer was greatly reduced and hence not sufficient to provide an adequate coating. As a result, creping aids were developed in order to supplement the coating formation.
The first creping aids utilized with through-air drying were animal hide glues. These glues, made from gelatin, have a complex, irregular molecular structure and dry to an irregular, disordered mass. Given their irregular molecular structure, these type of glues tend to form a coating on the surface of the Yankee dryer which exhibits low mechanical strength thus making the coating relatively easy to remove from the Yankee dryer surface. However, a major limitation of these glues is that they require large amounts of water in the fiber in order for the glues to achieve the necessary tack required at the pressure roll/Yankee nip. Thus for those paper machines which run at higher speed and lower consistencies, the effectiveness of these types of glues tends to be limited.
It is important that the creping aid allow for a proper balance between adhesion of the fibrous structure to the drying surface and the release of the fibrous structure at the creping blade. Historically, one of the difficulties encountered with the use of creping aids has been a tendency for the creping aid to form a bond between the fibrous structure and the drying surface at the point of creping such that the fibrous structure does not properly release from the drying surface. This results in portions of the fibrous structure remaining adhered to the surface thus causing defects in the fibrous structure and/or causing the fibrous structure to break.
Another common problem associated with the use of creping aids, is an excessive build-up of the creping aid on the drying surface. While some amount of buildup of the creping aid on the surface is essential, excessive buildup can produce streaks which impact the profile of adhesion across the width of the drying surface. This can result in bumps or wrinkles in the finished paper. Quite often, a second blade, known as a cleaning blade, is placed on the drying surface just after the creping blade. The purpose of the cleaning blade is to remove excess creping aid and other residue left behind. Historically, both the creping blade and cleaning blade have had to be frequently changed in order to prevent excessive buildup.
In order to prevent excessive buildup on the drying surface, it is important that the creping aid be rewettable. "Rewettable", as used herein, refers to the ability of the creping aid remaining on the Yankee dryer surface to be activated by the moisture contained in the fibrous structure. A marked increase in tack is indicative of high rewettability. Because through-air drying tends to remove more water than a conventional wet press section, a fibrous structure which is through-air dried will typically be transferred to the Yankee dryer at a higher consistency than a fibrous structure which is conventionally wet pressed.
Because a through-air dried fibrous structure contains less moisture than a similar fibrous structure which is conventionally wet pressed, there is less moisture available in the through-air dried fibrous structure to be transferred to the surface of the Yankee dryer. A fibrous structure which has been dried to a relatively higher fiber consistency, such as a through-air dried fibrous structure, is more difficult to adhere to the drying surface because the fibrous structure has less water available to rewet the creping aid at the point at which the fibrous structure is transferred to the drying surface.
With the advent of new through-air technology, such as that taught in commonly assigned U.S. Pat. Nos. 5,274,930, issued to Ensign et al. on Jan. 4, 1994 and U.S. Pat. No. 5,584,126, issued to Ensign et al. on Dec. 17, 1996 both of which are incorporated herein by reference, the distribution of moisture in the fibrous structure is more uniform throughout the fibrous structure for a given consistency. This results in less moisture in the knuckle area as measured after the fibrous structure leaves the through-air dryers. Knuckles which are dryer require that the coating formed by the application of the creping aid to the Yankee dryer provide good adhesion of the fibrous structure to the Yankee dryer as the fibrous structure contacts the Yankee dryer at the pressure roll.
U.S. Pat. No. 3,926,716, issued to Bates on Dec. 16, 1975 and incorporated herein by reference, teaches a polyvinyl alcohol creping adhesive which forms a film upon application to the surface of the Yankee dryer. It is believed that the polyvinyl alcohol exhibits a rewet mechanism wherein the film already on the surface of the Yankee dryer is rewetted as moisture is released from the fibrous structure at the pressure roll nip of the Yankee dryer.
Polyvinyl alcohol creping adhesives require less moisture than animal glue to provide tack to the surface of the Yankee dryer. However, since polyvinyl alcohol films are high molecular weight films having a highly crystalline structure, when used alone as creping adhesives, they tend to adhere strongly to the Yankee dryer surface resulting in excessive coating buildup and streaks.
Mixtures of polyvinyl alcohol and animal glue provide good tack and acceptable doctorability. However, it has been found that given the typically varying conditions of the paper mill water chemistry, the paper machine fiber and additive chemistry, and variations between different paper machines, it is desirable to blend polyvinyl alcohol with a variety of synthetic polymers. A common drawback observed with existing coating formulations is the occurrence of coating upsets (i.e.; any type of change that occurs to the coating which negatively impacts the coating characteristics) that result in poor reliability (eg. increased sheet breaks, flying and weaving, frequent blade changes, etc.).
With the advent of the addition of wet strength resins to the wet end of the paper machine based on polyamide polyamine epichlorohydrin (PAA) wherein the amine is a secondary amine, it was observed that adhesion of the fibrous structure to the Yankee dryer surface improved. This led to the development of creping aids based on similar PAA secondary amine resin chemistry as that used for wet strength resins. PAA secondary amine based creping aids have had wide acceptance in those paper machine systems using a conventional wet press section. However, in those paper machine systems utilizing through-air drying, creping aids based on PAA secondary amine chemistry have not enjoyed the same success.
This stems from the fact that creping aids based on PAA secondary amine resin chemistry are thermosetting and thus will cure on the heated surface of the Yankee dryer. Creping aids containing thermosetting resins are problematic in that the coating formed by the application of the creping aid to the Yankee dryer on a machine utilizing through-air drying tends to be brittle and exhibits poor adhesion to the dryer surface.
Another problem stems from those processes that utilize the wet end addition of wet strength resins which are thermosetting. The thermosetting wet strength resins will actively crosslink with creping aids which contain a secondary amine backbone. This results in the formation of a hard coating on the surface of the Yankee dryer having poor adhesive properties which in turn leads to an overall reduction in the efficacy of the creping aid.
U.S. Pat. No. 4,501,640, issued to Soerens on Feb. 26, 1985, purports to teach a method for creping a cellulosic fibrous structure wherein an admixture of polyvinyl alcohol and a water-soluble thermosetting cationic polyamide resin are applied to a creping cylinder.
U.S. Pat. No. 5,187,219, issued to Furman, Jr. on Feb. 16, 1993 purports to teach a creping adhesive composition comprising a thermosetting water soluble acrylamide polymer having glyoxal-reacted amide substituents and unsubstituted amide groups and a method of applying the composition.
U.S. Pat. No. 5,494,554, issued to Edwards et al. on Feb. 27, 1996, purports to teach the application of a creping adhesive comprising a thermosetting cationic polyamide resin and a plasticizer to the surface of a dryer.
All of these teachings suffer from a common drawback in that each of the purported creping aids contains a thermosetting resin. The thermosetting resin will cure on the heated surface of the Yankee dryer forming a brittle coating with poor adhesion characteristics. Additionally, each of the purported creping aids contains secondary amines. The use of creping aids containing secondary amines in conjunction with the addition of a thermosetting wet strength resin will tend to reduce the efficacy of the creping aid as the creping aid will crosslink with the active secondary amine groups present in the thermosetting wet strength resin. This results in a loss of adhesion of the creping aid.
The present invention provides a method of producing a creped paper product exhibiting overall improved crepe quality.
The present invention also provides a creping aid which exhibits overall improved performance in conjunction with all types of fibrous structures including those through-air dried fibrous structures transferred to the Yankee dryer at higher consistencies such as those fibrous structures taught in commonly assigned U.S. Pat. No. 3,301,746, issued to Sanford et al. on Jan. 31, 1967; U.S. Pat. No. 5,274,930 issued to Ensign et al. on Jan. 4, 1994; and U.S. Pat. No. 5,584,126, issued to Ensign et al. on Dec. 17, 1996 all of which are incorporated herein by reference.
This invention further provides a more efficient method for producing creped paper products wherein the creping aid is not deleteriously impacted by the use of thermosetting wet strength resins.
Benefits of the present invention include the ability to control coating buildup on the surface of the Yankee dryer as a function of changing paper machine system conditions and the ability to control individual properties of the coating such as the adhesive, release, and rewettability properties thereby providing enhanced machine runnability and improved paper quality.